This invention relates to an electromagnetic relay and, in particular, relates to an electromagnetic relay suitable for controlling a three-phase motor adapted to be mounted on an automobile.
In recent years, automotive power steering has been shifting from the hydraulic type to the electric type for the purpose of improvement in fuel consumption. Most electric power steering systems are of a DC motor driven type, but those of a three-phase motor driven type have been increasing following the application thereof to large-displacement vehicles. In case of the electric power steering system driven by the three-phase motor, a switch is required for simultaneously controlling the three phases. For example, an electromagnetic relay is useful which simultaneously controls the three phases at the neutral point (joining point) of the star connection for controlling the three-phase motor. In this case, the electromagnetic relay of this type is required to have high current-carrying performance and high interruption performance, and is further required to be small in size.
Conventionally, the control of the three-phase motor has been carried out by the following first or second method. In the first method, use is made of a plurality of electromagnetic relays each having a pair of a movable contact portion and a fixed contact terminal. A movable spring is used for forming the movable contact portion such that the movable spring is provided with a movable contact and an extraction terminal. Further, the plurality of electromagnetic relays are mounted on a circuit board. In the second method, a three-phase control electromagnetic relay is used. The three-phase control electromagnetic relay comprises two fixed contact terminals each having a fixed contact, and two movable contact portions for simultaneously shorting the two fixed contact terminals. A movable spring is used for forming the two movable contact portions such that the movable spring is provided with two movable contacts and one extraction terminal. A three-phase control is performed by the use of the three terminals, i.e. the extraction terminal and the two fixed contact terminals. Note that each of the fixed contact terminals of the electromagnetic relays used in the first and second methods is produced by processing a plate-like base member.
In the first method, inasmuch as the plurality of electromagnetic relays are used, the electromagnetic relays account for the large ratio on the circuit board. This goes against customers' requests for saving as much space as possible.
On the other hand, in case of the electromagnetic relay used in the second method, a difference exists in conductor resistance values between the two fixed contact terminals and between one of the two fixed contact terminals and the extraction terminal so that values of the current flowing between the three terminals differ from each other during on-operation of the electromagnetic relay. Further, when it is necessary to increase the current-carrying capacity, the fixed contact terminal should be increased in sectional area thereof. However, inasmuch as the fixed contact terminal has the plate-like shape, it can not be efficiently arranged on a base, which precludes reduction in size of the electromagnetic relay.
Moreover, in order to decrease a conductor resistance value from the movable contact to the extraction terminal via the movable spring in each of the electromagnetic relays used in the first and second methods, there is no alternative but to increase the sectional area of the movable spring. However, the movable spring is subjected to a restriction in terms of a required spring constant, and therefore, the sectional area thereof is limited in size in this regard.